\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}\sum_{n=0}^\infty \frac{x^n}{n!}	

\begin{matrix} \underbrace{ a+b+\cdots+z } \\ 2 \end{matrix}\end{equation*}
\end{document}
